French Dip Sliders with Pastrami and Caramelized Onions

Ingredients

• 1 Tbsp olive oil
• 2 Tbsp unsalted butter
• 1 large or 2 medium yellow onions, thinly sliced
• 1/4 tsp sugar
• 1 oz au jus mix, prepared according to package instructions
• 3 cups water, to prepare au jus sauce
• 16 Hawaiian sweet rolls
• 1/4 cup mayonnaise
• 1 lb deli pastrami or roast beef, thinly sliced
• 9 slices mild provolone cheese
• 2 Tbsp unsalted butter, melted (plus extra for greasing baking sheet)
• 1 Tbsp sesame seeds